Detection of spoofed satellite signals

ABSTRACT

A technology is provided for detecting spoofed satellite signals. An aircraft may be equipped with a top antenna that is adapted to receive satellite signals and a bottom antenna that is adapted to receive terrestrial signals. A signal may be received on the top antenna and the bottom antenna. Relative signal strength between the top antenna and the bottom antenna may be used to differentiate satellite sourced signals from terrestrial sourced signals.

BACKGROUND

The number of global and regional satellite communication and navigationsystems has grown rapidly in recent years among military, civil andcommercial users around the world. In general, satellite-basedcommunication and navigation systems provide location and timeinformation. Typically, satellite communication and navigation systemsmay function anywhere on or near the Earth where there is anunobstructed line of sight to one or more satellite. Thus, satellitesignals may not be received inside buildings, at subterranean locations,or underwater. Satellite communication and navigation systems have avariety of applications on land, at sea, and in the air. For example,satellite communication and navigation systems may facilitate activitiessuch as banking, mobile phone operations, navigation of commercialaircraft and boats, scientific studies, tracking, and surveillance.

There is growing reliance of aircraft on satellite communication,navigation and information (e.g., weather) systems. Satellite signalsmay be spoofed to disrupt aircraft operations and/or to crash theaircraft. For example, a spoofed satellite navigation signal may attemptto cause an aircraft to fly off course. An ability to authenticatesatellite signals protects against spoofing. Some techniques toauthenticate satellite signals may rely on unique characteristics of thesignals, and therefore cannot be applied to all satellite signals. Forexample, techniques used to authenticate a navigation signal may notwork for a communication signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the invention will be apparent from thedetailed description which follows, taken in conjunction with theaccompanying drawings, which together illustrate, by way of example,features of the invention; and, wherein:

FIG. 1 is an example illustration of an aircraft receiving satellitesignals from a satellite source and a terrestrial source in accordancewith an embodiment of the present invention.

FIG. 2 is an example system for determining that a satellite signalreceived from a signal source is an authentic satellite signal inaccordance with an embodiment of the present invention.

FIG. 3A is an example of a more robust system for determining that asatellite signal received from a signal source is an authentic satellitesignal in accordance with an embodiment of the present invention.

FIG. 3B illustrates a graphical representation for determining anunknown state, an authenticated state and a rejected state of asatellite signal in accordance with an embodiment of the presentinvention.

FIG. 3C illustrates a graphical representation for determining asatellite signal or a terrestrial signal in accordance with anembodiment of the present invention.

FIG. 4 is an example graphical representation of an antenna response foran upper antenna and a lower antenna receiving satellite signals inaccordance with an embodiment of the present invention.

FIG. 5 is an example graphical representation of satellite signals inrelation to terrestrial signals received at an aircraft in level flightin accordance with an embodiment of the present invention.

FIG. 6 is an example graphical representation of satellite signals inrelation to terrestrial signals received while an aircraft is banking inaccordance with an embodiment of the present invention.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result.

An initial overview of technology embodiments is provided below and thenspecific technology embodiments are described in further detail later.This initial summary is intended to aid readers in understanding thetechnology more quickly but is not intended to identify key features oressential features of the technology nor is it intended to limit thescope of the claimed subject matter.

There is a growing reliance of aircraft on satellite communication,navigation and information (e.g., weather) systems. Satellitecommunication and navigation systems include, but are not limited to,global positioning system (GPS), Global Navigation Satellite System(GLONASS), Galileo, Indian Regional Navigational Satellite System(IRNSS), Compass, Beidou, Doppler Orbitography and Radio-positioningIntegrated by Satellite (DORIS), and Quasi-Zenith Satellite System(QZSS). Spoofing of satellite navigation and communication signals maypose a threat to aviation safety. Other techniques to authenticatesatellite signals may rely on unique characteristics of navigationsignals, and therefore cannot be applied to all satellite signals. For anumber of reasons (e.g. ease of deployment, ease of concealment),spoofed satellite signals are most likely broadcast from terrestrialtransmitters.

Satellite communication or navigation signals may be spoofed to disruptaircraft operations and/or to crash the aircraft. In general, asatellite system spoofing attack, such as a global positioning system(GPS) spoofing attack, may attempt to deceive a satellite receiver bybroadcasting a spoofed signal with greater signal strength than comparedto the satellite signal. The spoofed signal may cause the satellitereceiver to determine an incorrect location. In other words, the spoofedsignal may communicate a false location to the satellite receiver. As anexample, the spoofed signal may falsely communicate a position that is afew degrees off of the satellite receiver's actual position. As aresult, the satellite receiver may falsely determine that the aircraftis off course, even though the aircraft is traveling in the correctorientation. The aircraft may adjust its orientation to correspond withthe spoofed signal, thereby causing the aircraft to travel off course.Satellite spoofing may trick a navigation system by communicatingspoofed signals, whereas satellite jammers may prevent satellitereceivers from receiving satellite signals altogether.

The threat of spoofed satellite signals may be mitigated by usingencrypted satellite signals. Since the parties that attempt to disruptthe satellite communication may not have knowledge of the properencryption keys, the spoofed signals may be distinguished from theauthentic satellite signals. However, cryptographic techniques may bedifficult to implement in civil aviation services because of thechallenges related to distributing and authenticating keys. Furthermore,existing services may not support cryptographic techniques forauthenticating satellite signals.

In general, aircraft operations may be threatened by spoofed signalsfrom a terrestrial transmitter. In other word, the party attempting todisrupt the aircraft operations may be based on the ground, rather thanthe sky. Although it is possible that spoofed signals may becommunicated from a satellite source, the likelihood of the spoofedsignals being communicated from the satellite source may be low whencompared to the likelihood of the spoofed signals being communicatedfrom the terrestrial transmitter.

An antenna that is shadowed by a structure of an aircraft, such as body,wings, or tail, may receive a more attenuated signal than an antennathat is not shadowed. When an aircraft has one or more antennas on boththe upper surface and lower surface of the aircraft, the strength of thesignal received via the upper and lower antennas may be compared todetermine if the source of the signal is above or below the aircraft.Signals from satellites may generally be received stronger via antennason the upper surface of an aircraft. Signals from terrestrial sourcesmay generally be received stronger via antennas on the lower surface ofan aircraft. This can be used to authenticate legitimate satellitesignals and reject spoofed satellite signals from a terrestrial source.

In one example, the source of the signals may be determined byinstalling two satellite antennas and two satellite signal receivers onthe aircraft. In one configuration, more than two satellite antennas andtwo satellite signal receivers may be installed on the aircraft.

In one exemplary embodiment, a first antenna used to receive signals maybe installed in proximity to the top of the aircraft. A second antennamay be installed in proximity to the bottom of the aircraft. In analternative configuration, the first antenna and the second antenna maybe installed in proximity to various locations on the surface of theaircraft, such as the top of the aircraft, the bottom of the aircraft,the wings of the aircraft, etc. In an alternative configuration, thefirst antenna and the second antenna may be installed on the tail of theaircraft or the nose of the aircraft. In yet another alternativeconfiguration, the first antenna and the second antenna may be installedin proximity to various locations on the surface of the vehicle.

The first antenna and the second antenna may receive signals whether thesignal originates from a satellite or from a terrestrial source.However, due to the directional characteristics of the antennas andmasking by the aircraft body, the observed signal strengths between theantennas may vary depending on whether the antenna is pointing towardsthe source of the signal or away from the source of the signal. Thesignals may have a respective signal strength. In one example, thesignal strength may be directly determined by a satellite receiver. Inan alternative configuration, the signal strength may be determinedusing a signal to noise ratio (SNR). The SNR may be defined as the ratioof signal power to the noise power, and is expressed in decibels (dB).

The signal strengths associated with the first and second antennas,respectively, may be compared. In general, if the signal strength fromthe top antenna is greater than the signal strength from the bottomantenna, then the source signal may likely be a satellite source, andindicate a non-spoofed signal. In general, if the signal strength fromthe top antenna is less than the signal strength from the bottomantenna, then the source signal may likely be a terrestrial source(i.e., indicating that the signals may be spoofed).

FIG. 1 is an example illustration 100 of an aircraft 120 receivingsatellite signals 140 from a satellite source 110 or a terrestrialsource 130. The terrestrial source 130 may be a terrestrial spoofingdevice that broadcasts spoofed signals, from ground level, to theaircraft 120. The aircraft 120 may include a top antenna 150 and abottom antenna 152. The top antenna 150 may be located in an upperregion of the aircraft 120 and the bottom antenna 152 may be located ina lower region of the aircraft 120. In alternative configurations, theaircraft 120 may include more than two antennas located in proximity tovarious regions of the aircraft (e.g., the bottom of the aircraft 120,the top of the aircraft 120, the front of the aircraft 120, the back ofthe aircraft 120).

When signals 140 are received by the aircraft from the satellite source140, the signals 140 received at the top antenna 150 may have strongersignal strengths as compared to the signals 140 received at the bottomantenna 152. In other words, the bottom antenna 152 may receive thesignals 140 from the satellite source 110 at the lower signal strengthbecause the aircraft's body may partially impede the communication ofthe signals to the bottom antenna 152. Similarly, when signals 140 arereceived by the aircraft from the terrestrial source 130, the signals140 received at the bottom antenna 152 may have stronger signalstrengths as compared to the signals 140 received at the top antenna150. In other words, the aircraft's body may partially impede thecommunication of the signals from the terrestrial source 130 to the topantenna 150, thereby resulting in the lower antenna strength at the topantenna 150 as compared to the bottom antenna 152. In variousembodiments, different criteria may be used for determining whether asignal is an authentic satellite signal or a signal from a terrestrialspoofer

FIG. 2 is an example system 200 for determining that a signal is anauthentic satellite signal 214. In one example, the system 200 may beincluded in an aircraft. The antennas 202 and 204 may be configured toreceive satellite signals from a satellite source or a terrestrialsource. The antennas 202 and 204 may be coupled to receivers 206 and 210(e.g., satellite receivers), respectively. In one example, the antenna202 may be located in an upper portion of an aircraft (e.g., the roof ofthe aircraft) and the antenna 204 may be located in a lower portion ofan aircraft (e.g., an underside or “belly” of the aircraft).

The receivers 206 and 210 may determine the signal strengths associatedwith the first and second satellite signals. The signal strengthreceived from the upper antenna may be compared with the signal strengthreceived from the lower antenna (as shown in block 208). In thisembodiment, a spoofed signal may be indicated when the signal strengthfrom the upper antenna is less than some predetermined threshold abovethe signal strength from the lower antenna. The defined threshold may beset to cover the highest upper/lower ratio a spoofed signal may achieve(i.e., the ratio when the aircraft is banking towards the source of thespoofed signal). In addition, the predefined threshold may include adynamic threshold based on at least one of: aircraft attitude, aircraftposition, aircraft heading, and satellite position. Since low elevationsatellite signals and satellite signals when the aircraft is banked awayfrom the satellite may have lower gain, these satellite signals may notbe above the defined threshold and therefore, whether the signal isspoofed may be undeterminable. However, high elevation satellitesignals, such as from geosynchronous navigation or communicationsatellites, may be authenticated when the aircraft is banking using thedefined threshold. As shown in the example in FIG. 6, the threshold foran aircraft in a 30° bank may be 10 dB. Since the cases of spoofedsignals are below the 10 dB threshold, the risk of falselyauthenticating a spoofed signal from a terrestrial source may beminimal. When the aircraft is banked away from the satellite, alegitimate satellite signal indicated by the elevation angles may not beauthenticated when the satellite signal falls below the 10 dB threshold.This type of system may be adequate when the system only needs toauthenticate high elevation satellite signals, such as geosynchronoussatellites.

If the signal strength received from the upper antenna is greater by thepredetermined threshold than the signal strength received from the lowerantenna, then the signal may be enabled (as shown in block 212) as anauthenticated satellite signal 214 being received from a satellitesource.

FIG. 3A is another example system 300 for determining that a signal isan authentic satellite signal. The satellite receiver 306 may be coupledto an antenna 302. The satellite receiver 308 may be coupled to anantenna 304. In one example, the antenna 302 may be located in an upperportion of an aircraft (e.g., the roof of the aircraft) and the antenna304 may be located in a lower portion of an aircraft (e.g., an undersideor “belly” of the aircraft).

An expected satellite signal strength module 310 may use informationabout the antennas, aircraft position, aircraft heading, aircraftattitude, satellite position, and surrounding terrain to determinethresholds used by the signal strength comparison module 312.

In one configuration, the attitude of the aircraft may be used by anexpected satellite signal strength module 310 to compute a satellitesignal threshold. The satellite signal threshold may be the smallestwhen the aircraft is flying level and increasing as the aircraft pitchesand/or banks. The smaller thresholds may allow valid satellite signals(e.g. signals from lower elevation satellites) to be authenticated.

In another configuration, aircraft position (e.g., geographicalcoordinates of the aircraft), aircraft heading and attitude, and asatellite position may be identified. This information may be used tocalculate a direction (i.e. azimuth and elevation angles) of thesatellite signal. The direction may be used in conjunction with a knownantenna reception pattern to calculate the gain or loss of the satellitesignal for both the upper and lower antennas with some threshold fornormal variances. In this configuration, an expected satellite signalstrength module 310 may compute a satellite signal threshold from theaircraft position, heading and attitude (i.e., pitch and bank of theaircraft), the position of the satellites, and antenna receptionpatterns. As an example, the satellite signal threshold for a satelliteat a particular elevation may be read from the Satellite line in thegraph of FIG. 5 for an aircraft in level flight or from the Satelliteline in the graph of FIG. 6 for an aircraft in a 30 degree bank towardsor away from the satellite.

In another configuration, the effective radiated power of the satellitesignal and propagation losses may be identified. In this configuration,an expected satellite signal strength module 310 may compute expectedsatellite signal strengths from the aircraft position, heading andattitude (i.e., pitch and bank of the aircraft), the position of thesatellite, expected satellite effective radiated power, propagationlosses, and antenna reception patterns.

The expected satellite signal strength module 310 may also compute aterrestrial signal threshold from aircraft attitude and perhaps aircraftaltitude and height of surrounding terrain (i.e. where a terrestrialsource might be located). As an example, the terrestrial signalthreshold may be computed from the Terrestrial line in the graph of FIG.5 for an aircraft in level flight or from the Terrestrial lines in thegraph of FIG. 6 for an aircraft in a 30 degree bank.

The first and second signals may be communicated to a signal strengthcomparison module 312. In one configuration, the signal strengthcomparison module 312 may compare the expected satellite signalstrengths with the measured signal strengths. If the expected satellitesignal strengths agree within a predefined threshold when compared withthe measured signal strengths, the spoofed signal determination module314 may indicate that the received signal is an authentic signalreceived from a satellite source. In another configuration, the signalstrength comparison module 312 may compare the ratio of received signalstrengths with the satellite signal threshold. If the ratio of receivedsignal strengths passes the satellite signal threshold, the spoofedsignal determination module 314 may indicate that the received signal isan authentic signal received from a satellite source. If the ratio ofmeasured signal strengths passes the terrestrial signal threshold, thenthe spoofed signal determination module 314 may determine that thesignal is a spoofed signal being communicated from a terrestrial source.

As shown in FIG. 3B, the signal authentication may include possiblestates of: Rejected (i.e. a spoofer), Unknown, and Authenticated (i.e. alegitimate satellite signal). When a signal is first acquired it has anauthentication status of Unknown. The authentication status changes toAuthenticated when the received signal strengths agree with the expectedsignal strengths or the signal ratio received from the upper and lowerantennas passes the satellite signal threshold. The authenticationstatus changes to Rejected when the signal ratio received from the upperand lower antennas passes the terrestrial signal threshold. Theauthentication status changes to Unknown when the received signal ratiois between thresholds and the receiver has lost lock on the signal. Theauthentication status is unchanged when the received signal ratio isbetween thresholds and the receiver has maintained lock on the signal.Thus, a satellite signal can stay in Authenticated state in cases wherethe aircraft banks away from the source of the satellite signal simplybecause the signal ratio received from the upper and lower antennasreasonably matches the ratio expected even though the ratio does notpass the satellite signal threshold.

As shown in FIG. 3C, the received signal may be determined to be asatellite signal or a terrestrial signal. For example, if the receivedsignal is above a satellite signal threshold, then the received signalmay be a confirmed satellite signal. If the received signal is below aterrestrial signal threshold, then the received signal may be aconfirmed terrestrial signal. Otherwise, the received signal may be anindeterminate signal and stays in current status if the receiver hasmaintained lock on the signal or changes to unknown status if thereceiver has not maintained lock on the signal.

FIG. 4 is an example graphical representation of an antenna response foran upper antenna and a lower antenna receiving satellite signals. Theantenna response may indicate the gain for a signal received by theantenna from a defined direction. In this example the direction isrepresented in degrees and the power level in decibels (dB).

In particular, FIG. 4 illustrates an expected antenna response for theupper level (i.e., when the antenna is mounted on the top of theaircraft), the antenna response for the lower level (i.e., when theantenna is mounted on the bottom of the aircraft), and a ratiorepresenting the difference between the antenna response for the upperlevel and the antenna response for the lower level. The ratio mayrepresent the signal strength measured by the receiver processing asignal from the upper antenna divided by signal strength measured by thereceiver processing the same signal from the lower antenna. In oneexample, satellite sources may have a signal strength ratio greater than1 or 0 dB and terrestrial sources may have a signal strength ratio ofless than 1 or 0 dB.

FIG. 5 is an example graphical representation of satellite signals inrelation to terrestrial signals received at an aircraft in straight andlevel flight. FIG. 5 illustrates an expected antenna ratio (i.e., theupper signal divided by the lower signal) when the aircraft receivessignals from a satellite source and when the aircraft receives signalsfrom a terrestrial source. The expected antenna ratio (in dB) may varydepending on the elevation angle of the signal source. In general, theelevation angle is the angle between the horizontal plane and the lineof sight, measured in the vertical plane. The elevation angle ispositive above the horizon (0° elevation angle), but negative below thehorizon.

As shown in FIG. 5, it may be difficult to distinguish satellite signalsthat are close to the horizon from terrestrial signals that are close tothe horizon. However, very low elevation satellite signals have poorersignal to noise ratios and may be more susceptible to variations inpropagation and reception. As a result, very low elevation satellitesignals may be poor sources of communication or navigation signals. Inaddition, terrestrial signals close to the horizon may be much furtherfrom the aircraft and therefore require much higher power to be strongerthan legitimate satellite signals than terrestrial signals in closerproximity to the aircraft. Therefore, the satellite signals that areclose to the horizon being less distinguishable from the terrestrialsignals that are close to the horizon may be an acceptable limitation.

FIG. 6 is an example graphical representation of satellite signals inrelation to terrestrial signals received when the attitude of anaircraft changes from level, such as when the aircraft is turning orbanking. In particular, FIG. 6 illustrates an expected antenna ratio(i.e., the upper signal divided by the lower signal) from a satellitesource in relation to a terrestrial source for an aircraft in a 30°bank.

In one example, satellite sources may be differentiated from terrestrialsources when the satellite signal curve is above the peak of the twoterrestrial signal curves (i.e., the “terrestrial-along-track” curve andthe “terrestrial-cross-track” curve). The peak of the terrestrial signalcurves may be used so that information on the location of theterrestrial signals may be unnecessary.

As shown in FIG. 6, the “terrestrial-along-track” signal curveillustrates that, for a spoofed signal source in line with the path theaircraft is flying, the signals received by the aircraft may beunaffected by the banking until the aircraft turns and the source is nolonger in line with the aircraft's flight path. The“terrestrial-cross-track” signal curve illustrates that the upper/lowerratio increases for spoofed signal sources in the direction of theaircraft bank, as well as the decreases for spoofed signal sourcesopposite the direction of the aircraft bank. The converse may be truefor changes in aircraft pitch with “along track” affected by pitch and“cross track” not affected by pitch.

As shown in FIG. 6, satellite signals close to the horizon opposite thedirection the aircraft is banking may fall below 0 dB, and terrestrialsignals (i.e., the “terrestrial-along-track” signal curve) close to thehorizon in the direction that the aircraft is banking may rise above 0dB. In addition, terrestrial signals along the axis of the bank (i.e.,the “terrestrial-cross-track” signal curve) may be unchanged. Inaddition, high elevation (i.e. near zero) satellite signals andsatellite signals from the direction of the bank may be authenticated,but not satellite signals opposite the direction of the bank. Forexample, satellite signals may be differentiated from terrestrialsources when satellite signals less than 15° above the horizon andterrestrial sources farther from the aircraft nadir than the aircraftaltitude (i.e., elevation angles greater than −45°) are excluded.

In accordance with one embodiment of the present invention, a method fordetecting spoofed satellite signals is disclosed. The method cancomprise: receiving a signal via a first antenna, the first antennalocated on an upper portion of an aircraft; receiving the signal via asecond antenna, the second antenna located on a lower portion of theaircraft; comparing a first signal strength associated with the signalreceived via the first antenna with a second signal strength associatedwith the signal received via the second antenna; and determining thatthe signal is a spoofed satellite signal being communicated from aterrestrial source when the second signal strength is greater than thefirst signal strength.

In one aspect, the method further comprises using the signal receivedvia the first antenna when the first signal strength associated with thesignal received via the first antenna is greater than the second signalstrength associated with the signal received via the second antenna. Inaddition, the first antenna and the second antenna receive the samesignal from a signal source, the signal source including at least oneof: a satellite source and the terrestrial source.

In one aspect, the method further comprises comparing the first signalstrength associated with the signal and the second signal strengthassociated with the signal with a dynamic threshold; and determiningthat the signal is the spoofed satellite signal being communicated froma terrestrial source based on the dynamic threshold. In addition, themethod further comprises determining that the signal is the spoofedsatellite signal being communicated from the terrestrial source when theaircraft is banked at a defined angle based on the first signal strengthin relation to the second signal strength.

In accordance with one embodiment of the present invention, anadditional method for detecting spoofed satellite signals is disclosed.The method can comprise: receiving a signal via a plurality of antennas,wherein the plurality of antennas are positioned on an aircraft; anddetermining that the signal is a spoofed satellite signal based on acomparison of the signal strength received via each of the plurality ofantennas. In one example, the plurality of antennas are positioned on anupper portion or a lower portion of the aircraft.

In one aspect, the method can further comprise performing the comparisonusing an aircraft attitude to determine an expected relative signalstrength between the plurality of antennas for an authentic satellitesignal. In addition, the method can comprise performing the comparisonusing at least one of aircraft position, aircraft heading, aircraftattitude, and satellite position to determine an expected relativesignal strength between the plurality of antennas for an authenticsatellite signal.

In an additional aspect, the method can further comprise performing thecomparison using an aircraft attitude to determine an expected relativesignal strength between the plurality of antennas for the spoofedsatellite signal from a terrestrial source. In addition, the method canfurther comprise performing the comparison using at least one ofaircraft position, aircraft heading, aircraft attitude, and informationon surrounding terrain to determine an expected relative signal strengthbetween the plurality of antennas for the spoofed satellite signal froma terrestrial source.

In one aspect, the method can further comprise determining that thespoofed satellite signal is from a terrestrial signal source. Inaddition, the method can further comprise using a dynamic threshold fordetermining that the signal is the spoofed satellite signal from theterrestrial source or an authentic satellite signal, wherein the dynamicthreshold is based on at least one of: aircraft attitude, aircraftposition, aircraft heading, and satellite position.

In an additional aspect, the method can further comprise determining anauthenticated state when relative signal strengths associated with thesignal meets the dynamic threshold for confirming that the signal is theauthentic satellite signal. In addition, the method can further comprisedetermining a rejected state when relative signal strengths associatedwith the signal meets the dynamic threshold for confirming that thesignal is the spoofed satellite signal from the terrestrial source.Furthermore, the method can further comprise determining an unknownstate when relative signal strengths associated with the signal does notmeet the dynamic threshold for confirming that the signal is theauthentic satellite signal nor for confirming that the signal is thespoofed satellite signal from the terrestrial source.

Various techniques, or certain aspects or portions thereof, may take theform of program code (i.e., instructions) embodied in tangible media,such as floppy diskettes, CD-ROMs, hard drives, or any othermachine-readable storage medium wherein, when the program code is loadedinto and executed by a machine, such as a computer, the machine becomesan apparatus for practicing the various techniques. In the case ofprogram code execution on programmable computers, the computing devicemay include a processor, a storage medium readable by the processor(including volatile and non-volatile memory and/or storage elements), atleast one input device, and at least one output device. The volatile andnon-volatile memory and/or storage elements may be a RAM, EPROM, flashdrive, optical drive, magnetic hard drive, or other medium for storingelectronic data. The base station and mobile station may also include atransceiver module, a counter module, a processing module, and/or aclock module or timer module. One or more programs that may implement orutilize the various techniques described herein may use an applicationprogramming interface (API), reusable controls, and the like. Suchprograms may be implemented in a high level procedural or objectoriented programming language to communicate with a computer system.However, the program(s) may be implemented in assembly or machinelanguage, if desired. In any case, the language may be a compiled orinterpreted language, and combined with hardware implementations.

It should be understood that many of the functional units described inthis specification have been labeled as modules, in order to moreparticularly emphasize their implementation independence. For example, amodule may be implemented as a hardware circuit comprising custom VLSIcircuits or gate arrays, off-the-shelf semiconductors such as logicchips, transistors, or other discrete components. A module may also beimplemented in programmable hardware devices such as field programmablegate arrays, programmable array logic, programmable logic devices or thelike.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of executable code may, forinstance, comprise one or more physical or logical blocks of computerinstructions, which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of executable code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.The modules may be passive or active, including agents operable toperform desired functions.

Reference throughout this specification to “an example” means that aparticular feature, structure, or characteristic described in connectionwith the example is included in at least one embodiment of the presentinvention. Thus, appearances of the phrases “in an example” in variousplaces throughout this specification are not necessarily all referringto the same embodiment.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary. In addition, various embodiments and example of the presentinvention may be referred to herein along with alternatives for thevarious components thereof. It is understood that such embodiments,examples, and alternatives are not to be construed as defactoequivalents of one another, but are to be considered as separate andautonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided, such asexamples of layouts, distances, network examples, etc., to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, layouts, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

What is claimed is:
 1. A method for detecting spoofed satellite signals,the method comprising: receiving a signal via a first antenna, the firstantenna located on an upper portion of an aircraft; receiving the signalvia a second antenna, the second antenna located on a lower portion ofthe aircraft; comparing a first signal strength associated with thesignal received via the first antenna with a second signal strengthassociated with the signal received via the second antenna; anddetermining that the signal is a spoofed satellite signal beingcommunicated from a terrestrial source when the second signal strengthis greater than the first signal strength.
 2. The method of claim 1,further comprising using the signal received via the first antenna whenthe first signal strength associated with the signal received via thefirst antenna is greater than the second signal strength associated withthe signal received via the second antenna.
 3. The method of claim 1,wherein the first antenna and the second antenna receive the same signalfrom a signal source, the signal source including at least one of: asatellite source and the terrestrial source.
 4. The method of claim 1,further comprising comparing the first signal strength associated withthe signal and the second signal strength associated with the signalwith a dynamic threshold; and determining that the signal is the spoofedsatellite signal being communicated from a terrestrial source based onthe dynamic threshold.
 5. The method of claim 1, further comprisingdetermining that the signal is the spoofed satellite signal beingcommunicated from the terrestrial source when the aircraft is banked ata defined angle based on the first signal strength in relation to thesecond signal strength.
 6. A method for detecting spoofed satellitesignals, the method comprising: receiving a signal via a plurality ofantennas, wherein the plurality of antennas are positioned on anaircraft; and determining that the signal is a spoofed satellite signalbased on a comparison of the signal strength received via each of theplurality of antennas with a dynamic threshold.
 7. The method of claim6, further comprising using the dynamic threshold for determining thatthe signal is the spoofed satellite signal from the terrestrial sourceor an authentic satellite signal, wherein the dynamic threshold is basedon at least one of: aircraft attitude, aircraft position, aircraftheading, and satellite position.
 8. The method of claim 6, furthercomprising performing the comparison using an aircraft attitude todetermine an expected relative signal strength between the plurality ofantennas for an authentic satellite signal.
 9. The method of claim 6,further comprising performing the comparison using at least one ofaircraft position, aircraft heading, aircraft attitude, and satelliteposition to determine an expected relative signal strength between theplurality of antennas for an authentic satellite signal.
 10. The methodof claim 6, further comprising performing the comparison using anaircraft attitude to determine an expected relative signal strengthbetween the plurality of antennas for the spoofed satellite signal froma terrestrial source.
 11. The method of claim 6, further comprisingperforming the comparison using at least one of aircraft position,aircraft heading, aircraft attitude, and information on surroundingterrain to determine an expected relative signal strength between theplurality of antennas for the spoofed satellite signal from aterrestrial source.
 12. The method of claim 6, wherein the plurality ofantennas are positioned on an upper portion or a lower portion of theaircraft.
 13. The method of claim 6, further comprising determining thatthe spoofed satellite signal is from a terrestrial signal source. 14.The method of claim 6, further comprising determining an authenticatedstate when relative signal strengths associated with the signal meetsthe dynamic threshold for confirming that the signal is the authenticsatellite signal.
 15. The method of claim 6, further comprisingdetermining a rejected state when relative signal strengths associatedwith the signal meets the dynamic threshold for confirming that thesignal is the spoofed satellite signal from the terrestrial source. 16.The method of claim 6, further comprising determining an unknown statewhen relative signal strengths associated with the signal does not meetthe dynamic threshold for confirming that the signal is the authenticsatellite signal nor for confirming that the signal is the spoofedsatellite signal from the terrestrial source.
 17. A system for detectingspoofed satellite signals, the system comprising: a receiving moduleconfigured to receive a signal via a first antenna and via a secondantenna, wherein the first antenna is located on an upper portion of anaircraft and the second antenna is located on a lower portion of theaircraft; a comparison module configured to compare a first signalstrength associated with the signal received via the first antenna witha second signal strength associated with the signal received via thesecond antenna; and a signal module configured to determine that thesignal is a spoofed satellite signal being communicated from aterrestrial source when the second signal strength is greater than thefirst signal strength.
 18. The system of claim 17, wherein the signalmodule is further configured to use the signal received via the firstantenna when the first signal strength associated with the signalreceived via the first antenna is greater than the second signalstrength associated with the signal received via the second antenna. 19.The system of claim 17, wherein: the comparison module is furtherconfigured to compare the first signal strength associated with thesignal and the second signal strength associated with the signal with adynamic threshold; and the signal module is further configured todetermine that the signal is the spoofed satellite signal beingcommunicated from the terrestrial source based on the dynamic threshold.20. The system of claim 19, wherein the dynamic threshold is based on atleast one of: aircraft attitude, aircraft position, aircraft heading,and satellite position.